1. Field of the Invention
The present invention relates to an electroacoustic transducer and an electronic apparatus including the electroacoustic transducer. More particularly, the present invention relates to an electroacoustic transducer having a structure in which magnets are provided both above and below a diaphragm, and also relates to an electronic apparatus including such an electroacoustic transducer.
2. Description of the Background Art
Recently, in the field of portable electronic apparatuses, such as a mobile telephone and a personal digital assistant (PDA), reduction in thickness and power consumption of an electronic apparatus has been accelerated. As in the case of the electronic apparatus, an electroacoustic transducer included in the electronic apparatus is demanded to reduce its thickness while achieving more efficient power consumption. Accordingly, in order to realize reduction in thickness and power consumption, an electroacoustic transducer as described below has been proposed.
FIG. 16 illustrates the structure of a conventional electroacoustic transducer. In the conventional electroacoustic transducer illustrated in FIG. 16, a casing 20 includes a circular cover 1 and a circular frame 2 joined to the cover 1. Each of the cover 1 and the frame 2 is open on one end. The cover 1 includes a plurality of holes 11 for emitting sound provided in a circle. A magnet 3 is fixed on an inner plane of the cover 1 such that the center axis of the cover 1 passes through the center of the magnet 3. A disc-like diaphragm 4 is provided within the casing 20 so as to provide space G between a lower surface of the magnet 3 and the diaphragm 4. The diaphragm 4 is secured at its outer circumferential portion sandwiched between the cover 1 and the frame 2. A drive coil 5 is fixed on a lower surface of the diaphragm 4 so as to have the same center axis as the center axis of the magnet 3. An electrode 6 for applying an electric current to the drive coil 5 is fixed on the bottom of the frame 2. A lead line (not shown) extending from the drive coil 5 is connected to an end of the electrode 6.
In the conventional electroacoustic transducer illustrated in FIG. 16, the magnet 3 emits magnetic fluxes from its lower surface, such that magnetic fluxes emitted from the vicinity of the center of the magnet 3 pass substantially perpendicularly through the drive coil 5, while magnetic fluxes emitted from an outer circumferential portion of the magnet 3 radiate from the lower surface of the magnet 3 so as to pass diagonally through the drive coil 5. In a magnetic field formed by emission of the above-described magnetic fluxes, when an electric current is applied to the drive coil 5, a drive force in a direction perpendicular to the diaphragm 4 is generated in the drive coil 5. Such a drive force causes the diaphragm 4 to vibrate up and down, thereby producing sound. The conventional electroacoustic transducer illustrated in FIG. 16 is configured to emit magnetic fluxes directly from the magnet 3. Accordingly, this conventional electroacoustic transducer requires neither a yoke nor a center pole, and therefore the entire thickness thereof can be reduced. Moreover, the drive coil 5 has a high degree of freedom in the range of possible winding widths, and therefore has a high degree of freedom in the range of possible impedance values. Accordingly, by increasing the impedance of the drive coil 5, it is made possible to achieve reduction in power consumption of the conventional electroacoustic transducer.
Further, in the conventional electroacoustic transducer illustrated in FIG. 16, the drive force generated in the drive coil 5 increases in proportion to the intensity of magnetic fluxes perpendicular to a direction of the electric current flowing through the drive coil 5 and a vibration direction of the diaphragm 4. In FIG. 16, magnetic fluxes parallel to the vibration direction of the diaphragm 4 are dominant over the magnetic fluxes perpendicular to the vibration direction of the diaphragm 4. Accordingly, the conventional electroacoustic transducer illustrated in FIG. 16 is not able to obtain a satisfactory drive force, and therefore is able to provide only a low reproduced sound pressure.
Furthermore, the intensity of the magnetic fluxes emitted from the magnet 3 decreases in proportion to the distance from the magnet 3. Accordingly, the drive force generated in the drive coil 5 varies between the case where the diaphragm 4 is located in a downward direction from its initial position as shown in FIG. 16 (i.e., a direction away from the magnet 3) and the case where the diaphragm 4 is located in an upward direction from its initial position (i.e., a direction toward the magnet 3). Such a variation of the drive force causes distortion of the drive force in the conventional electroacoustic transducer illustrated in FIG. 16, resulting in deterioration of reproduced sound.